Field of the Invention
The present invention relates to a catalyst for producing 1,1-difluoroethane (CHF2CH3, hereinafter referred to as the xe2x80x9cHCFC-152axe2x80x9d) and producing method thereof. More particularly, it is to provide the catalyst prepared by impregnating palladium on the active carbon pretreated with an aqueous hydrogen fluoride solution and an aqueous hydrogen chloride solution in series and its use in the production of 1,1-difluoroethane by dehydrochlorinating 1,1-difluoro-1-chloroethane (CF2ClCH3, hereinafter referred to as the xe2x80x9cHCFC-142bxe2x80x9d) at 240-300xc2x0 C. in the supplying ratio of 2-6 (H2/HCFC-142b) with maximizing a selectivity toward the product of HCFC-152a.
Chlorofluorocarbons (hereinafter referred to as the xe2x80x9cCFCsxe2x80x9d) were widely used for refrigerants, foaming agents, cleaning solvents, and aerosol propellants due to its non-toxicity and stabilities against chemicals and heat However, since it has been reported that CFCs are released into stratospheric layer and their chlorine atoms decomposed by UV light destroy the earth""s ozone layer, the current concern over the use of CFCs and their effect on the environment has been emphasized. According to memorandum of Montreal, the developed countries have prohibited producing CFCs and imports and exports since 1996 and the developing countries also reduce the use of CFCs up to 50% by 2005 and will be disused by 2010. There has been an increasing demand for producing hydrochlorofluorocarbons (hereinafter referred to as the xe2x80x9cHCFCsxe2x80x9d) by replacing chlorine atoms by hydrogen or fluorine atoms as alternatives for CFCs used as refrigerants, foaming agents and the like. However, even if HCFCs are decomposed faster than CFCs, they are still environmental hazard materials because they also contain chlorine atoms in the molecules.
On the other hand, the development for manufacturing processes of hydrofluorocarbons (hereinafter referred to as xe2x80x9cHFCsxe2x80x9d) as excellent alternatives for CFCs has been rapidly increased due to its advantages in no harming earth""s environment and ozone layer.
Of manufacturing processes, hydrogenolysis of CFCs provides HFCs and especially hydrogenolysis in the presence of a catalyst is proved as a very useful method for producing HFCs. Examples for producing HFCs from CFCs or HCFCs are HFC-125(CF3CF2H) from CFC-115(CF3CF2Cl), HFC-32(CH2F2) from CFC-12(CF2Cl2), and HFC-152a(CHF2CH3) from HCFC-142b(CF2ClCH3).
Japan Patent No. 3-83940 discloses a method for producing HFC-152a by hydrogenolysis of CFC-142b at 300-340xc2x0 C. in the presence of a catalyst prepared by impregnating a metal (platinum, palladium, rhodium, nickel, or a mixture thereof) on an active carbon. The yield of the reaction is reported as 80% but its reactivity and selectivity are not mentioned. And the lifetime of the catalyst which is one of the most important matters in hydrogenolysis of halogen-containing compounds has not been mentioned, either. In general, if the temperature for hydrogenolysis is higher than 300xc2x0 C., sintering of a metal catalyst can occur and its activity rapidly decreases and this is proved in the comparison examples disclosed in Japan Patent No. 7-126197. Thus, in order to solve these problems Japan Patent No. 7-126197 discloses an addition of bismuth on the palladium catalyst to enhance the activity and selectivity. In the hydrogenation of HCFC-142b with hydrogen (molar ratio of hydrogen:HCFC-142b=4:1) performed at over 300xc2x0 C. in the presence of palladium and bismuth, a reactivity of HCFC-142b was 80% and a selectivity toward HFC-152b was 86%, while the activity was reduced to 30% in the absence of bismuth after 10 hrs of reaction. However, the reaction temperature is relatively high over 300xc2x0 C. and it produces C2H5F over 10% as a by-product.
There have been attempts to prepare improved methods for preparing palladium catalyst (Pd/C) such as pretreatment of an active carbon or addition of platinum type metal to palladium. Pretreatments of an active carbon with acid or alkali in the hydrogenation of CFC and HCFC are disclosed in U.S. Pat. No. 5,136,113, Wiersma (A. Wiersma, E. J. A. X. van de Sandt, M. Makkee, H. van Bekkum, and J. A. Moulijn, Catal. Today, 27, (1996) 257), and Albers (P. Albers, R. Burmeister, K. Seibold, G. Prescher, S. F. Parker, and D. K. Ross, J. Catal. 181, (1999) 145). It can be expected that the pretreatment of an active carbon can reduce impurities within the active carbon and enhance distribution of a metal catalyst and thus, improve its activity. However, experiments are essentially required to find better pretreatment method and preparation method of a catalyst and to maximize reaction conditions.
Palladium supported on an active carbon (Pd/C) is widely used commercial catalyst for hydrogenation. However, if the reaction temperature is low in the hydrogenation of CFC and HCFC, the reactivity becomes poor and if it is high, an activity rapidly decreases due to sintering of palladium catalyst. It is required to obtain the preparing method of a catalyst and appropriate reaction conditions because sintering can be affected by used support, preparing method and reaction temperature. And further, it is essentially required to enhance the selectivity toward dechlorination in dehalogenations of dechlorination and defluorination.
As a result of that inventors of the present invention have investigated various pretreatment methods of active carbon as the support of palladium catalyst (Pd/C) used in the dehydrochloriation of HCFC-142b, the inventors have realized that the pretreatment of an active carbon with two acids sequentially affects the activity and selectivity of the catalyst. In the present invention the catalyst prepared by impregnating palladium on the pretreated active carbon with hydrogen fluoride solution and hydrogen chloride solution in series is used in the dehydrochlorination reaction of HCFC-142b to HFC-152a.
The present invention is characterized by a catalyst prepared by impregnating palladium on the active carbon pretreated with hydrogen fluoride solution and hydrogen chloride solution sequentially and a producing method of 1,1-difluoroethane by dehydrochlorination of 1,1-difluoro-1-chloroethane in the presence of said catalyst.
The present invention is described in detail as set forth hereunder.
The present invention is characterized by the catalyst prepared by impregnating palladium on the active carbon pretreated with hydrogen fluoride solution and hydrogen chloride solution sequentially. That is, the active carbon is treated with hydrogen fluoride solution in an appropriate concentration, washed, and retreated with hydrogen chloride solution.
In the process of pretreatment of the active carbon, both hydrogen fluoride solution and hydrogen chloride solution are used in the concentration of 0.1-2 mol/l. The mixture is stirred for more than 5 hrs, washed with distilled water till pH reaches to 4-7, preferably 6, and dried. Palladium chloride (PdCl2) as a precursor of palladium (Pd) is dissolved in aqueous hydrogen chloride solution and then impregnated on the dried active carbon. The impregnated catalyst is dried at 50-200xc2x0 C. and calcinated at 300-500xc2x0 C. under atmospheric conditions to obtain the desired palladium catalyst.
The palladium catalyst (Pd/C) is used in hydrogenolysis of HCHC-142b and optimal reaction conditions such as temperature and molar ratio of reactants are investigated.